1. Field of the Invention
The present invention relates generally to a method for cutting a wafer protecting sheet, particularly to a method for cutting an obverse protecting sheet that is used for a semiconductor wafer when machining with a back grinder and the like.
2. Description of Related Art
FIG. 4 is a sectional side view of a semiconductor wafer 1, on which a protecting sheet 2 is adhered and cut in a conventional cutting method. Semiconductor devices such as ICs are formed on the obverse of the wafer 1 and are individually inspected, and the wafer 1 is then diced with a dicing saw to separate devices. The semiconductor devices for IC cards and the like are required to be thin. In order to produce the devices of a desired thickness, the reverse of the wafer 1 is ground as shown in FIG. 5 with a back grinder (not shown) before the wafer 1 is diced into the separate devices with the dicing saw. FIG. 6 is a sectional side view of the semiconductor wafer 1 that has been ground as shown in FIG. 5.
Before the reverse of the wafer 1 is ground, the protecting sheet 2 is adhered on the obverse of the wafer 1 to protect the devices, which have been formed on the obverse. The protecting sheet 2 is composed of a plastic base 3 and an adhesive layer 4. The protecting sheet 2 is adhered to the obverse of the wafer 1 through the adhesive layer 4, and is then cut along the outer periphery of the wafer 1 with a cutting tool 5 as shown in FIG. 4.
The semiconductor wafer 1 can generate micro cracks and fragments during manufacturing processes if the edge at the periphery of the wafer 1 is sharp. Then, the micro cracks generated at the edge can develop toward the inside of the wafer 1 and the micro fragments can have a bad effect on the wafer 1, and the wafer 1 can be damaged as a result. In consideration of this, the edge portion at the outer periphery of the wafer 1 is chamfered beforehand to eliminate the sharp edge.
In a case to adhere the protecting sheet 2 on the obverse of the chamfered wafer 1, the protecting sheet is adhered not only to the level part of the obverse, but also to the chamfered inclined surface. Then, the protecting sheet 2 is conventionally cut with the cutting tool 5′ in this state along the periphery of the wafer 1 at the right angle with respect to the level part of the obverse of the wafer 1 as shown in FIG. 4. The cut face and the level surface of the protecting sheet 2 form the right angle.
Then, the wafer 1 is mounted and fixed with the reverse up and the obverse, over which the protecting sheet 2 is adhered, down on a worktable (not shown) of the back grinder, and the reverse of the wafer 1 is ground with a grindstone (not shown) so that the wafer 1 becomes a predetermined thickness.
As an extremely thin IC card such as a smart card has been started, the semiconductor devices to be incorporated thereinto have been required to be extremely thin, 30 μm to 50 μm for example. In this case, the wafer 1 must be ground so that a finishing thickness is extremely thin, and more than half of the original thickness of the wafer 1 is removed as shown in FIG. 5, where a reference numeral 6 represents a portion that is ground and removed. At this time, not only the wafer 1 of brittle material but also the plastic base 3 and the adhesive layer 4 of the protecting sheet 2 are partly ground as shown in FIG. 5, and the ground plastic and adhesive material sticks on grinding particles (e.g. diamond particles) of the grindstone and thereby clogs up the grindstone. Then, machining performance is deteriorated, and machining efficiency and quality of the machined products are lowered; also, the wafer 1 is damaged.